body{
  margin:0;
  padding:0;
}
/*第一层结构：flex*/
body{
  display: flex;
  flex-direction: column;
}
body #head{
  height: 100px;
  background-color: teal;
}
body #section{
  flex: 1;
  /*display: flex;*/
  overflow: hidden;
}
body #section .active{
  background-color: gray;
}
body #content{
  height: 100%;
  overflow-y: scroll;
  padding: 20px;
}